1. Field of the Invention
The present invention relates to a distributor, a synthesizer and an S/N enhancer. More particularly, the present invention relates to an S/N enhancer for improving the signal to noise (S/N) ratio of a main signal contained in an input signal, for example, and relates further to a distributor and a synthesizer for use in such an S/N enhancer.
2. Description of the Related Art
An example of a conventional S/N enhancer is disclosed in Japanese Unexamined Patent Publication No. 4-123502. FIG. 9 is a schematic view illustrating an example of such a conventional S/N enhancer. An S/N enhancer 1 shown in FIG. 9 comprises an input terminal 2 which is connected to the input end of a directional coupler 3. This directional coupler 3 is used to divide a signal input to the input terminal 2 so as to output a high-level signal of the same level as that of the input signal and a low-level signal attenuated by, for example, approximately 30 dB. The respective components of the input signal, the high-level output signal and the low-level output signal will be discussed below in more detail.
The two output terminals of the directional coupler 3 are respectively connected to the input terminals of two magnetostatic wave filters 4a and 4b utilizing a surface magnetostatic wave mode. The magnetostatic wave filters 4a and 4b have a YIG (yttrium iron garnet) thin film formed as a ferromagnetic substrate on one of the main surfaces of a GGG (gadolinium gallium garnet) substrate, an input-side transducer and an output-side transducer in the shape of single lines, which are arranged in parallel spaced from each other on the YIG thin film, and a DC magnetic-field applied to the YIG thin film in a direction parallel to the direction in which these transducers extend.
The magnetostatic wave filters 4a and 4b have a frequency-selective non-linear amplitude limitation characteristic which is the same for both filters. This frequency-selective non-linear amplitude limitation characteristic will be explained based on the following example.
Now, a signal of a frequency f.sub.1 and a signal of a frequency f.sub.2 are inputted into a filter at the same time.
1) If both signals do not exceed a saturation level, then both are outputted from the filter without amplitude limitation.
2) If the signal of frequency f.sub.1 does not exceed the saturation level and the signal of frequency f.sub.2 exceeds the saturation level, then the signal of frequency f.sub.1 is outputted from the filter without amplitude limitation, and on the other hand, the signal of frequency f.sub.2 receives amplitude limitation before it is outputted from the filter.
3) If the signal of frequency f.sub.1 exceeds the saturation level and the signal of frequency f.sub.2 does not exceed the saturation level, then the signal of frequency f.sub.1 receives amplitude limitation before it is outputted from the filter, and on the other hand, the signal of frequency f.sub.2 is outputted from the filter without amplitude limitation.
4) If both signals exceed the saturation level, then both are outputted from the filter after receiving amplitude limitation.
The magnetostatic wave filter 4a is used as a limiter for limiting the amplitude of a high-level main signal which is contained in the high-level signal output from the directional coupler 3. The other magnetostatic wave filter 4b is used to allow the low-level signal output from the directional coupler 3 to pass.
The output end of the magnetostatic wave filter 4b is connected to the input end of a 180.degree. shifter 5. This 180.degree. shifter 5 is used to invert the phase of a signal output from the magnetostatic wave filter 4b. Further, the output end of the magnetostatic wave filter 4a and the output end of the 180.degree. shifter 5 are respectively connected to the two input ends of a directional coupler 6. This directional coupler 6 is used to attenuate the level of the signal output from the magnetostatic wave filter 4a and synthesize that signal with the signal output from the 180.degree. shifter 5. Further, the output end of the directional coupler 6 is connected to an output terminal 7.
Therefore, in the S/N enhancer 1, in the section between the input terminal 2 and the output terminal 7, the directional coupler 3, the magnetostatic wave filter 4a and the directional coupler 6 constitute a first signal path, and the directional coupler 3, the magnetostatic wave filter 4b, the 180.degree. shifter 5 and the directional coupler 6 constitute a second signal path.
In this S/N enhancer 1, an input signal containing a high-level main signal, and low-level noise whose frequency is different from the main signal, is input to the input terminal 2, and is divided by the directional coupler 3 into a high-level signal at nearly the same level as that of the input signal and a low-level signal which is attenuated by, for example, approximately 30 dB. In this case, the high-level signal contains a high-level main signal and low-level noise whose frequencies differ from each other, and the low-level signal contains a low-level main signal and still-lower-level noise whose frequencies differ from each other.
In the magnetostatic wave filter 4a, the main signal in the high-level signal receives amplitude limitation because its level is high; however, the noise in the high-level signal does not receive amplitude limitation because its frequency differs from the frequency of the main signal and because its level is low. In contrast to this, in the other magnetostatic wave filter 4b, since the levels of both the main signal and the noise in the low-level signal are low, they do not receive amplitude limitation. (However, the levels of both the high-level signal and the low-level signal are slightly attenuated due to insertion loss in the magnetostatic wave filters 4a and 4b.)
The phase of the signal output from the magnetostatic wave filter 4b is inverted by the 180.degree. shifter 5. The level of the signal output from the magnetostatic wave filter 4a is then attenuated by the directional coupler 6, and that level-attenuated signal and the signal output from the 180.degree. shifter 5 are synthesized. In this case, the phase of the signal output from the magnetostatic wave filter 4b is inverted so that the phases of the noise in the two signals synthesized by the directional coupler 6 become opposite to each other. Therefore, the noise which passes through the first signal path including the magnetostatic wave filter 4a and the noise which passes through the second signal path including the magnetostatic wave filter 4b cancel each other. Further, the main signal which passes through the first signal path receives amplitude limitation by the magnetostatic wave filter 4a, whereas the main signal which passes through the second signal path does not receive amplitude limitation by the magnetostatic wave filter 4b. Therefore, the value of the main signal level which passes through the first signal path is reduced by the value of the main signal level which passes through the second signal path, and the resultant value of the main signal is obtained at the output end of the directional coupler 6 or the output terminal 7. Therefore, in the S/N enhancer 1, S/N of the input signal is improved.
FIG. 10 is a schematic view illustrating another example of a conventional S/N enhancer. In the S/N enhancer shown in FIG. 10, as compared with the S/N enhancer shown in FIG. 9, the directional coupler 3 and directional coupler 6 are not used. Instead, the input terminal 2 is directly connected to the input end of the magnetostatic wave filter 4a and connected to the input end of the magnetostatic wave filter 4b via an attenuator 8 formed of two resistors, and the output end of the magnetostatic wave filter 4a is connected to the output terminal 7 via an attenuator 9 formed of two resistors, and the output end of the 180.degree. shifter 5 is directly connected to the output terminal 7.
In the S/N enhancer shown in FIG. 10, the magnetostatic wave filters 4a and 4b operate in the same way as the S/N enhancer shown in FIG. 9, whereby the S/N ratio of an input signal is improved.
In the S/N enhancer shown in FIG. 9, since the directional couplers 3 and 6 are used to distribute or synthesize a signal, it is difficult to achieve a small size. On the other hand, in the S/N enhancer shown in FIG. 10, since the directional couplers are not used and the attenuators 8 and 9 formed of resistors are used instead, it is easy to achieve a small size. However, since the attenuators 8 and 9 are directly connected to the input terminal 2 and the output terminal 7, respectively, achievement of impedance matching between the input terminal 2 and an external circuit and between the output terminal 7 and an external circuit is difficult, so that an undesired signal due to a magnetostatic wave reflected to the output-side transducers in the magnetostatic wave filters 4a and 4b may pass through the attenuator 8, possibly causing a ripple to occur in the output signal.